Lithium ion batteries represent an important class of rechargeable energy storage in which lithium ions move from the negative electrode to the positive electrode during discharge. First proposed in 1972, lithium ion batteries have become widely used in the portable consumer electronics and have been extended into electric vehicles as well. Lithium ion batteries are popular for several reasons including their light weight when compared to most other rechargeable batteries and the fact that they have high open-circuit voltage, low self-discharge rate, reduced toxicity and lack battery memory effect.
In a lithium battery under load, the lithium ions stored on the anode migrate from the anode through an electrolyte medium to the cathode creating an electrical current. During the charging process the lithium metal migrates back onto the anode. Currently, graphite is often used as the anode material. While not necessarily the optimal anode material, graphite's high availability and low cost currently make it an attractive solution. When carbon is used as the anode, the reaction on a Li-ion cell is given as: 6C+2 LiCoO2LiC6+2 Li0.5CoO2. The reactions at each electrode are given as:At the cathode: LiCoO2−0.5Li+-0.5e−Li0.5CoO2143 mAh/gAt the anode: 6C+Li++e−LiC6372 mAh/g
One alternative to graphite as an anode material is silicon. The Li—Si anode system has one of the highest possible gravimetric capacities of all the elements. Further, unlike carbon based anode systems, silicon does not suffer from solvent co-intercalation breakdown. Silicon shows these advantageous properties due to the chemical structure of the Li—Si system—a single silicon atom is able to bind to 4.4 lithium ions, whereas it takes 6 carbon atoms to retain a single lithium ion. When silicon as an anode material is compared to graphitic carbon, the theoretical capacities differ by an order of magnitude. For a range of x from 0 to 4.4, the theoretical specific capacity of pure silicon is 4200 mAh/g, far greater than the theoretical capacity of 372 mAh/g for graphitic carbon. The full reaction is written as:4Si+15Li++15e−Li15Si4=>3580 mAh/g.
While the above-noted properties seem to make silicon an ideal anode material, one consequence of silicon's enhanced lithium ion interaction is a large increase in volume dilation (>400%). This volume dilation results in the silicon anode structure being subjected to high stress levels and mechanical breakdown. This breakdown means that traditional silicon anodes are unable to go through the multiple charge/discharge cycles necessary for commercialization. Hence, a critical unmet need for the use of silicon as a viable anode material is find a way to structurally stabilize it against multiple volume expansions.